Twin Bell Circuit Circuit

It is often desirable for a single doorbell to be operated by two buttons, for instance, one at the front door and the other at the back door. The additional button, S2 in series with the break contact of relay Rel, is connected in parallel with the original bell-push, SI. When S2 is pressed, the bell voltage is rectified by D1 and smoothed by CI. After a time, t = R{R2C2, the direct voltage across C2 has risen to a level here T1 switches on. Relay Rel is then energized and its contact breaks the circuit of S2 so that the bell stops ringing. After a short time, CI and C2 are discharged, the relay returns to its quiescent state and the bell rings again.

In this way, SI will cause the bell to ring continuously, wliile S2 makes it ring in short bursts, so that it is immediately clear which` button is pushed.